1. Field of the Invention
The present invention relates to a sheet feeding apparatus which is incorporated into an image forming system such as a copying machine, laser beam printer, facsimile and the like.
2. Related Background Art
Conventionally, as one of sheet feeding apparatuses incorporated into a sheet supplying portion of a printer, a sheet feeding apparatus having an arrangement, for example, as shown in FIGS. 11 and 12 has been known.
In FIGS. 11 and 12, a sheet supply roller 10 comprising a metal core 12 integrally formed with a support shaft 11 and a friction member 13 covering an outer peripheral surface of the metal core is disposed at a downstream side of a sheet stacking support 3 on which a plurality of sheets S are stacked. A separating pad 17 formed on a free end of a pivotable support member 81 is urged against the sheet supply roller 10 by a spring force of a separating spring 20.
Further, the sheet S is urged against the sheet supply roller 10 by means of a pressurizing means comprising an intermediate plate 7 attached to a support plate 80 and a pressure spring 9. On one end of the support shaft 11, there are arranged a cam 33, a gear 35 drivingly connected to a drive source (not shown), and a spring clutch 40. The spring clutch 40 comprises a control ring 36, a boss 37 and a spring (not shown), and one revolution of the spring clutch 40 is controlled by a solenoid 41.
A pair of feed rollers 83 comprising a drive roller and a driven roller urged against the drive roller are arranged within a body frame of the apparatus. Gears 30a, 30b and adapted to transmit a driving force and are integrally formed with the drive and driven rollers, respectively, to be meshed with each other.
When the solenoid 41 is turned ON, the engagement between a control pawl 41a of the solenoid 41 and a pawl 36a of the control ring 36 is released, thereby activating the spring clutch 40, with the result that a driving force of the gear 35 is transmitted to the support shaft 11 to rotate the sheet supply roller 10. Further, by the rotation of the cam 33, the regulation of a free end 6b of the support plate 6 is also released, with the result that the intermediate plate 7 is lifted by the pressure spring 9 to urge an uppermost sheet on the stacked sheets S (sheet stack) against the sheet supply roller 10, thus feeding out the sheet. When the sheet supply roller 10 is rotated by one revolution, the pawl 36a of the control ring 36 is engaged by the control pawl 41a of the solenoid 41 which is now in an OFF condition, thereby releasing the spring clutch 40 to stop the sheet supply roller 10. At the same time, the support plate 6 is forcibly lowered by the cam 33, with the result that the intermediate plate 7 is stopped and maintained in a lowered position. The sheet S separated by the separating pad 17 is conveyed by the paired rollers 83 while being guided by guide plates 21a, 21b.
However, in the above-mentioned conventional arrangement, the following problems arise.
Since the separating pad 17 and the sheet S are always urged against the sheet supply roller 10, even during the sheet supplying operation, static electricity is generated due to the sliding contact between the sheets S and is accumulated on the surface of the sheet, thus causing the poor transfer of toner to the sheet to worsen the image quality.
Further, as shown in FIG. 13, the pressure between the drive roller 23 and the driven roller 25 of the paired feed rollers 83 is not uniform, the pressures F at both end portions being stronger than the pressure f at an intermediate portion due to the flexion of the rollers. Accordingly, the pinching force between the rollers is also stronger at the both end portions than at the intermediate portion. Consequently, in the case of a sheet feeding apparatus of frictional separating type, unless a friction separating means is arranged at a central position with respect to a width of the sheet S, the sheet conveying force is unbalanced due to the back tension generated by the sheet supply roller 10, separating pad 17 and the sheet stacking support 3 during the sheet supplying operation, thereby easily causing the skew-feed of the sheet S. In particular, when the sheet supply roller 10 is constituted by a round or cylindrical roller, such back tension will be increased.
Further, as shown in FIGS. 14 and 15, in the case where a universal tray is used for stacking sheets S having different sizes, since a separating and supplying portion must be always arranged at a central position with respect to a width of the sheet S (referred to as "center reference" hereinafter), the spring force of the tension spring 29 for the paired feed rollers 83 must be selected with higher accuracy. Furthermore, in the center reference, since regulating plates 85a, 85b for regulating the transverse position of the sheets S stacked on the stacking means must be shifted symmetrically with respect to the center of the sheet S, the number of parts of the apparatus is increased, thus making the apparatus expensive and worsening the accuracy of the positioning of the sheets in the transverse direction.
Further, as shown in FIG. 12, when the sheet feeding apparatus 1 is replaced by another one, since the sheet supply roller 10 is disposed at the center of the sheet, it is difficult to maintain a space required for exchanging the sheet supply roller 10.